Spider Hunt Quest Problem?

I've had the quest "Spider Hunt" for quite awhile, and I got myself up to 11 spiders killed. Now I've been trying to get that one last spider to complete the quest and I can't. Is there a certain type of spider that I'm supposed to be defeating that isn't specified (the specification is "Spiders") or is this a quest/game malfunction. I think I've seen others with the same problem as me.